text:
What is the purpose of having a robotic arm fly a Boeing 737?
What is the aim of using a robotic arm instead of embedding the system on a plane? I read the 'Robotic Co-Pilot Autonomously Flies and Lands a Simulated Boeing 737' documentation but it didn't help me to understand.

See also "Otto Pilot"

That arm is lucky that there was no gusty side wind or other requirement for opeating the pedals.

The fundamental benefit is that the operator doesn't have to make a lot of adjustments on "legacy" airplanes, they can "simply" install the arm on the existing aircraft that they already have in service.
And this is the main selling point for such a device, they are not marketing it to operators that still have to buy or lease an airliner, but to those that already have one and might want to reduce the number of pilots they have to employ.
Gürkan Çetin notes in the comments that this device is not certified (yet). That's partially the point of such demonstrations, to convince authorities that it can be certified, and to incentivate operators to lobby for their approval.

I'd consider it more likely to be employed for unmanned test flights than for replacing a copilot. Think crash tests, which in the past were flown manned, with the crew parachuting out after setting the autopilot on a crash trajectory to impact on a specific point. Using this system the aircraft could be flown remotely for the duration.

@jwenting that's a good point that I did not consider. Why don't you write your own answer?

@Federico because I have no documentation about this concept, only an idea of how it could be employed.


text:
What gas is used in airships to give them buoyancy?
In the 1930s Zeppelins used hydrogen as the lighter-than-air gas to fill the envelope and give the ship buoyancy.
What kind of gas is used today?
Is it still as flammable as the Hydrogen from airships of the 1930s?

Helium. This is a trivial topic. Try using the Wikipedia before asking obvious questions.

inflammable mean flammable?

@TylerDurden Not just helium - as ratchet freak pointed out there are airships that use hot air.

@voretaq7 The OP specifically asked about "lighter-than-air gas". I presumed he was talking about substances not including air based on that.

There are only 2 lifting gasses that are lighter than air and practical for airships;

helium: this is a noble gas which means it's inert. Safest option all around except that US has a monopoly on the production of it. That is also the reason Zeppelins didn't use it because of an embargo the US had at the time against the Germans.
hydrogen: even more buoyant than helium but flammable. It is however easier to get by running a current through water or dropping some metals in an acid.

The third option for providing buoyancy is heated air, this requires a heat source to counteract the loss of heat over time.

In addition to the above, the current Zeppelin NT uses Helium - as probably do all (or almost all?) the currently flying commercial airships. See also Why the world is running out of helium

Only two gases which are lighter than air and suitable for airships? You seem to miss Ammonia, Methane or Neon - all of which are possible, although potentially impractical. Methane is combustable, Neon is very rare, Ammonia isn't particularly efficient being not much lighter than air - but those aren't necessarily showstoppers, since Helium is also very rare (in terms of practical availability) and hydrogen is combustible

@JonStory they are also much less buoyant than helium and hydrogen, so you would need much larger bags for the same amount of lift

Indeed, but you make it sound like those are the only two usable gases - perhaps the first sentence would be better as "There are only two gases that are both lighter than air, and practical for use in an airship", avoiding the implication that they are the the only ones suitable BECAUSE they're the only ones lighter than air

he did say 'practical' not 'usable'.

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text:
How can pressure altitude be calculated manually using hPa?
I am currently going through PPL and sitting for the meteorological paper and I see this question in the mock exam.

What is the approximate International Standard Atmosphere (ISA)
standard pressure at FL 180?

The answer has it as 500 mba but I can't seem to get the answer anywhere. What's the best formula I should use in this sort of questions?

I remember studying from an EASA book that used the rule of thumb where 1 mb=30’ change. That way, FL180 would have 413 mb. If the answer is 500mb, they took 1mb=35’. I used the an iPhone app based CX3, and it seems to be take 30’ as well. Strange!

@PapaMike99 the pressure gradient changes over altitude. At sea level, the gradient is approximatly 27 ft / mb, at FL180 it is approximately 48 ft / mb.

@DeltaLima I had totally forgotten about that! Thanks!

For altitudes in the troposphere, use this, which I give here in Excel format:
= 1013.25 * ( 1 - ( 0.0000068755856 * 18000 ) ) ^5.2558797
This yields a result of 505.99 mB at 18,000 feet.
The generalized form of this is:
= isoSLpressure * ( 1 - ( 0.0000068755856 * altFt ) ) ^5.2558797
Where:

isoSLpressure is your choice of any ISO sea level pressure (29.92", 1013.25mB, 760.0mm, or 14.696psi, they all work the same.)
altFt is altitude MSL in feet.

This formula basically calculates the pressure RATIO of the atmosphere at any given altitude, which is a different way of applying the pressure lapse rate.
All credit goes to Ed Williams.

I had a typo in my answer, was missing the second closing parenthesis in the formulas.  Fixed.

Hi Max, which Ed Williams are you referring to? Wikipedia gives me an actor and several football players. I also found a pilot boat but that is as close to aviation as I got.

@DeltaLima Ed has been the long time de facto keeper of the Aviation Formulary, which has been a goldmine for many years.  https://edwilliams.org/avform147.htm  Not unlike other references (like the CRC math tables) the formulary is not a tutorial and is of no use in learning the concepts.  But if you already understand the concepts and just need to find a formula or constant now and then, it's a treasure chest that you'll keep a link to the rest of your life.

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text:
How does varying camber affects control over an aeroplane?
How does a trailing edge flap actually help an aeroplane?
It just changes camber but what does that exactly do that’s beneficial?

Have you attempted any research at all?

Flaps will:

Increase coefficient of lift - more lift for same speed, which can get one off the ground faster - or - same lift at a lower speed.  This helps slow down for landing.

Increase drag - changes glide slope if your landing glide slope is "long".  Higher flaps settings are especially useful for this.

Now, the fun part:

Moves the center of pressure of the wing rearwards.  This will cause a pitching down tendency.

Causes a stronger downwash on the tail, creating a pitching up tendency.  This is especially noticeable flying a high wing/low tail aircraft such as a Cessna 172, particularly at the higher end of the Vfe range.

Designers try to balance the effects of 3. and 4., but dumping full flaps could help raise the nose in the event of an elevator failure just before landing.

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text:
How can I determine the home base of a German-flagged aircraft?
Is there any way (lookup on website, database etc. or other means) how to determine the home base of an aircraft, specifically for an aircraft registered in Germany.

As far as I know, there is no official registration of the main operating base of aircraft in Germany.
What you could do is use FlightRadar24.com or similar aircraft tracking website and investigate the history of flights to determine the airport it is most frequently operating at.


text:
Why is there variation in helicopter autorotation maneuvers?
Why are there some different autorotation techniques?
For example, on light helicopters, like the Airbus Helicopters H125 or Aérospatiale Gazelle, you do not flare too much: at 70-100ft AGL adjust some nose up attitude, and keep it almost constant until the touchdown, and cushion the touchdown with collective.
But on bigger helicopters, you have a lot of nose up on the beginning of the flare (up to 20-25 degrees nose up attitude),  and then the cyclic is moved forward to place the helicopter in the level fuselage attitude for landing, and then you use collective to cushion the touchdown.

Do you have a source that says the H125 and Gazelle need substantially less than a 20deg flare?

Yes, I have manuals

I'm not 100% sure, but this is likely due to the ratio of torque load to rotor inertia.   Bigger, multi-engine helos typically have more torque load per unit inertia, meaning they lose NR quicker on the collective pull.  A bigger flare boosts NR, guarding against excess loss of rotor speed before landing.


text:
RB-77C Destroyer on March 10th, 1964 over Magdeburg
It is claimed that a spy plane with the registration RB-77C Destroyer was shot down near Magdeburg on 10 March 1964. Does anyone know anything about this?

Re "Does anyone know anything about this?"-- without a doubt, "someone" does!  ; )

You have the correct date, and an almost correct aircraft designation; it was an RB-66C not a RB-77C:

On 10 March 1964, a 19th TRS RB-66C flying on a photo-reconnaissance mission from the Toul-Rosières Air Base in France, was shot down over East Germany by a Soviet Mikoyan-Gurevich MiG-21 after it had crossed over the border due to a compass malfunction. The crew ejected from the aircraft and, following a brief period of detention, were repatriated to the United States.
source: Wikipedia - Douglas B66

This was covered by the printed news media of the day:

WIESBADEN, Germany, March 21 — The Soviet Union released today an injured United States Air Force officer, one of three crewmen of an RB‐66 reconnaissance bomber shot down over East Germany 11 days ago.
The officer, First Lieut. Harold W. Welch, navigator of the plane, was turned over to United States authorities in Magdeburg, East Germany, and brought to the Wiesbaden Air Force Hospital. He suffered a broken arm and leg when he landed in a tree after having parachuted from the burning jet.
source: New York Times - March 22, 1964, Page 1


text:
Can I get an EASA Class 1 Medical with a cured depression?
I had depressions and from January this year to last week I took escitalopram 10mg daily.
I want to be a pilot and apply to the european flight academy at the end of 2025.
Is this a problem to get my medical?
Of course I cant hide my past but because there’s around half a year between the meds and the medical test is it realistic that I get a medical?
Thanks in advance!

This is similar to a question about PTSD.

Different question, same answer: https://aviation.stackexchange.com/q/84891/7532


text:
What is the effect of suction surface pressure coefficient on lift under flow separation?
Quick question. Lets say we are looking at the CP plot of the 13 degree angle of attack, seen in the image below:

Now, let's say we force separation right after that suction peak. This will mean we have a constant lower pressure than the fully attached case... and doesnt this mean we should have more lift?

Reason I ask is that I have heard that by intentionally forcing separation towards the rear of an airfoil, you can increase its aft loading and thus overall lift. So I was thinking that we could apply that near where the peak is to have lower pressure over a larger %chord.
For example, look at this cylinder which has separated flow. The Cp coefficient along the aft side is constant ~-0.5 for High Re and ~-1.3 for Low Re due to separation…

Or maybe I am answering my own question here, but if we force separation, then our suction peak will be significantly reduced as we only get the peak due to the pressure recovery - which is also seen in the cylinder photos
Edited to show Cp of a Cylinder with attached and separated flow.

Like sophit said... pretty much in a nutshell...

@ThomasPerry: But then how come for a cylinder with separated flow, the Cp stays constant when you have separation? https://www.aerodynamics4students.com/fluid-mechanics/cylinder_flow_pressures.gif

Your assumptions about suction peak and low pressure to the trailing edge are both wrong. https://aviation.stackexchange.com/questions/43221/what-is-the-effect-of-flow-separation-on-lift-pressure-distribution-and-drag explains that separation makes the airfoil behave as if it were longer, thicker and had less camber. This reduces the negative pressure on the rear of the airfoil, thus also reducing the suction peak. The resulting curve looks more like your fully separated flow curve, but at a lower angle of attack.

@aerokid95 There is no circulation.  The $C_p$ so encountered is within the turbulent wake.  The presence of circulation, particularly in the case of a cylinder, will largely preclude the development of separation. Note that when separation and stall occurs on an airfoil, circulation is lost and the $C_p$ rises in the wake similar to your illustration.  Excellent question!

Let's see how this works. Attachment of the flow through the region of pressure recovery is necessary to preclude stall. Here is the velocity profile of an airfoil designed to intentionally force separation to the trailing edge and, in fact, preclude separation altogether. Flow is from left to right in the illustration.   As air flows from the nose of the airfoil (left) toward the trailing edge (right), the velocity of the flow is accelerated to a minimum value of $C_p$.  If the Reynolds number is sufficiently high and the contour surface clean and smooth, this is a region of laminar flow. At the point where the velocity profile encounters pressure recovery, the flow begins to decelerate and becomes turbulent.  The characterization of the flow as laminar or turbulent specifically refers to the nature of the flow in the boundary layer of flow at the surface of the airfoil contour. When separation of the turbulent boundary layer occurs, the airfoil is said to stall.
In this region of pressure recovery, the upwardly concave portion of the velocity profile promotes turbulent boundary-layer attachment meaning that turbulent separation and consequent stall, is precluded.  This concave portion indicates that the abrupt deceleration of the flow is being relaxed as the flows progresses toward the trailing edge.  B.S. Stratford examined this property in the pressure recovery and attachment of turbulent flow along the wall of a wind tunnel where the wall of interest could be distorted to preclude turbulent separation.  Robert Liebeck investigated airfoil design using this property in the mid-1970s. He wanted to examine airfoils having significant lift properties due to delayed stall. F.X. Wortmann was well aware of this aspect regarding concavity in the recovery region of the velocity profile, and initially investigated airfoils designed with this recovery property in the early-to-mid 1950s. His airfoils have been used extensively in many high-performance sailplanes and human-powered aircraft.
Lift is proportional to the area enclosed within the velocity profile.  When separation occurs forward of the trailing edge up to the velocity peak near the leading edge, the velocity profile becomes roughly flat over a major portion of the airfoil chord at a $C_p$ value of -0.4, more or less.  Consequently, the area enclosed within the velocity profile is significantly reduced, meaning that airfoil lift is reduced. This is the consequence of stall.
To gain as much lift as possible means that the designer must make the enclosed area as large as possible thereby pushing the recovery portion as close to the trailing edge as possible. However, this reaches a practical limit.  Consequently, the design of wings having favorable lift characteristics typically have the placement of minimum pressure sufficiently forward to accommodate an adequate range in angles of attack while giving favorable stalling characteristics.

let's say we force separation right after that suction peak. This will mean we have a constant lower pressure than the fully attached case... and doesn't this mean we should have more lift?

Not really
If we force separation then (at least) three things would happen:

This artificially triggered "suction" would act on the rear portion of the airfoil which normally points upward and backward; the relevant resulting aerodynamic force would therefore also point upward - adding up to the lift as you guessed - but also backward - adding up to the drag.
In subsonic flows, what happens downstream influences also what happens upstream; this means that the stalled region on the backward part of the airfoil would change the forward distribution of suction, basically reducing its value: the $C_p$ there would reach maybe only -1 instead of the -4 of your picture.
The redistribution of $C_p$ on the backward part of the airfoil would increase the pitching moment generated by the airfoil, which would need to be counteracted increasing the download on the tail (at least on a conventional airplane) with a relevant increase in the so-called trim drag.

At the end you wouldn't obtain a higher lift rather a higher drag.

But how come, over here when you see flow separating on a cylinder, the Cp stays constant after separation? It doesn’t drop to 0. https://www.aerodynamics4students.com/fluid-mechanics/cylinder_flow_pressures.gif

@aerokid95 right, I didn't really explained myself well, let me rework my answer


text:
How does new media streaming technology equate to fuel cost increase?
In regards to this article on BBC, which discusses the use of streaming technology to allow passengers to watch films/TV shows etc. on their portable devices, the following statement caught my eye:

It would increase the drag of our aircraft by 2%, which equates to 20
  million euros [$23.6m; £15.5m] in extra fuel.

Is this cost increase purely to do with an increase in weight and additional drag owing to the new equipment required or is there any other factors at play here?

An antenna produces parasitic drag but also the weight of the equipment add to the induced drag a wing produces (the more weight the wing has to lift, the more drag the wing itself creates). But it's hard to believe even both make up 2%

@SteveH you might be surprised at how much difference a small aero change can make.

Because it says "drag", my guess is that implies an external antenna.
For example (which I found using a Google search),

Commercial and Large VIP Aircraft Broadband Internet Solutions
HR6400 Antenna System
The HR6400 antenna system for commercial and large VIP aircraft
provides passengers and crew true wireless broadband Internet access.
No matter where you fly - over land or water in any region of the
world - you can browse the Internet, send and receive emails in
real-time, use Voice-Over-IP (VoIP) telephony, access your VPN,
conduct video conferencing, and watch IP television.
The emerging standard for Ku-band systems with the most efficient
Ku-band antenna available, the HR6400 is mounted on the aircraft's
fuselage, providing all the performance you need in the smallest
possible footprint. The easy-to-install HR6400 comes complete with
antenna, antenna control unit, high power transceiver, radome, and
radome attachment ring. It covers everything from the energy coming
from the satellite to the modem connection.

For example, here is a typical antenna installation (not necessarily the one mentioned above) on a 737:

It is important to distinguish the streaming of media from the on-board server, and an internet connection to the outside world.
The first is just a matter of setting up an access point inside the cabin that wouldn't have any impact on the drag.
The latter requires some sort of antenna that probably needs to be placed outside to achieve a decent connection. There are many proposal for those, one for instance is here). Note that in the background section it says:

Current aeronautical antenna solutions have drawbacks in terms of cost (active phased arrays) or drag (dish antennas).

I guess that in the interview Mr Hurley was referring to those dishes, as other projects seems to have a definitely minor impact on an aircraft cross section.

The one's I have seen are dome shaped and don't produce much drag.

Does the antenna need to be placed outside the fuselage simply because they're usually aluminum? In other words, with a composite aircraft (like a 787) could you add this antenna without cutting a hole in the fuselage?

@Gabe: Skin of composite aircraft is still conductive to provide adequate protection from lightning (made so by embedding a wire mesh in the top layer). The antenna needs to go outside this. It could possibly go in the radome in the nose or another antenna bay with non-conductive cover could be designed somewhere, but that is only possible as part of the design, not as after-market addition.

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text:
Why/How does a "Powerflow" exhaust increase horsepower?
I've seen airplanes (typically Cessna 172s) with a "Powerflow" exhaust system and then claim to get several more horsepower, resulting in faster cruise speeds.  How does a Powerflow exhaust work, and why does it increase horsepower?

It has to do with resonance.  Think of a pipe organ - each pipe resonates at a certain frequency, which we hear as a note or tone.  The length of the pipe determines its note/tone/resonance frequency.  Your plane's exhaust system has a certain resonance frequency.
Your plane's engine has a certain note or tone, too.  That is largely determined by engine RPM.  A powerflow exhaust matches the exhaust resonance frequency with the engine's rpm at a typical high cruise setting.
On a micro level, here is what happens.  After a cylinder's power stroke, when the exhaust valve first opens, exhaust floods out into the exhaust system.  Soon the exhaust valve shuts and the inertia of the gasses moving down the exhaust system cause the exhaust gas to continue towards the exit, creating a lower pressure behind it (near the exhaust valve).  If the resonance frequency of the exhaust system matches the rpm, at that point the exhaust valve opens on the next cycle, and the lower pressure area in the exhaust assists in scavenging the exhaust out of the cylinder.  On the subsequent intake stroke, there is less residual pressure in the cylinder and it can "suck in" more fuel/air mixture.  More fuel/air, more power.
Another way to say this is that it takes power for the engine to force the exhaust out of the cylinder and through the exhaust system.  The Powerflow system reduces the power required to do this.

This is fascinating. I wondered why the exhaust pipe was typically lengthened when it had Powerflow installed. Maybe it's worth another question, but what is the procedure for finding the resonance frequency? And then do they do it so it's typically at cruise RPMs or take-off RPMs (since I haven't seen these on airplanes with variable speed propellers)?

My guess is that full power rpm is used to determine the exhaust system's resonance frequency. The marketing department wants to show maximum benefit!

Powerflow exhaust systems are based on the principle that an engine is, essentially, a large noisy air pump: air is drawn in through the intake, and pumped out the exhaust.
A limiting factor on how much horsepower an engine produces is how quickly you can move a given volume of air through the engine (because of restrictions in either the intake or exhaust system).
Some aircraft, like Cessna 172s or 140/150/160 horsepower Piper Cherokees, have an exhaust system that restricts how quickly exhaust gasses can be "pumped out", due to unfavorable geometry, narrow pipe sizes, or both.
Powerflow systems, like "tuned exhaust" systems you find on sports cars, provide a wide pipe and a geometry that encourages pulling as much of the exhaust gas out of the cylinder as possible, which means less energy is wasted pushing the piston up during the exhaust stroke, and more oxygen-rich air will be pulled into the cylinder during the subsequent intake stroke - both contributing to producing more usable horsepower.


text:
Incident over the pacific
I read this from the new CNN.com: Two planes nearly collide over Pacific.
The death toll would amount more than the Tenerife's one if things went actually wrong.
Kevin Townsend tells more in his post: Two Weeks Ago, I Almost Died in the Deadliest Plane Crash Ever.
My question is:
Can the passengers ask for some compensation (damages or whatever) due to the event?

What damages? All this is is media speculation about a crash that did not happen.

I dont see how you could claim compensation for not crashing. Mind you in the USA.......

Kevin Townsend invents more in his post fixed

I didn't tell asking compensation for not crashing. An extreme diving is frightening and passengers may not stand that. There should be someone else responsible for the incident and its eventual consequences.

I certainly hope not or the next time the pilots will be less inclined to take decisive action, thus aggravating the situation. The system worked, an error managed to slip through but was caught before it could turn into a disaster. 600 feet is not in any way an "extreme dive" unless you're 500 feet above the ground. Sure it might be frightening but I'm guessing that people would rather ride a little roller coaster than cutting it too close.

Assuming that the pilot has reacted properly (which is my opinion too) implies the ATC is to blame in the very case. I think of "Moral damage", a legal concept mostly known in countries whose legal system is based on the Code Napoléon: mainly civil suit but not criminal charge.

Not sure why the down votes here. Whether or not we agree there was an event, there is a single solidly defined question and it is directly related to a fight on an airplane. Are we saying (by voting down) that we don't like questions of airline responsibilities? If you hover your mouse over the down arrow itself, the tooltip states "the question does not show any research effort; it is unclear or not useful." The only part of that which might apply is "not useful" and I think it is.

I voted to close this question because it is, in essence, a legal question.  The fact that the answer would be primarily opinion based is somewhat secondary...  Either way I don't think this question is a good fit for this site.

CNN did it again. Absolutely no proof about a plane crash, and even their headline admitted it ("Plane drops 600 feet in 60 seconds"... since when has that been an emergency decent???)

If anyone's still interested, check out The Reality Behind the “Hawaii Near Collision” over at NYC Aviation

I am not a lawyer. None of this should taken as, or construed to be legal advice.
Looking over United's Contract of Carriage upon which their passenger bill of rights are based, I found this paragraph:

UA shall not be liable for any death, injury, delay, loss or other damage of whatsoever nature (hereafter referred to collectively as “damage”) arising out of or in connection with carriage or other services performed by UA, unless such damage is proven to have been caused by the sole negligence or willful misconduct of UA and there has been no contributory negligence on the part of the Passenger

One of the planes involved in the incident to which you refer was a UA flight. They were flying per ATC guidance, therefor there was no "sole negligence or willful misconduct" etc.
That being said, to answer your given question, yes the passengers can ask for compensation. They are unlikely to be granted any by the airline, as that would set a bad precedent (not a legal precedent, but one that might affect them legally down the road). They are under no obligation to make compensation, as they fulfilled the contract between themselves and the passengers. The passengers were safely delivered to their destination with little to no delay.
In my personal opinion, the airline owes nothing to any of the passengers, even for "mental anguish" as the total deviation from flight path was well within the amount that could be seen or felt due to severe turbulence.

Thank you for the answer! You've overlooked the "ATC" side of the question: Can it be also sued or just an eventual sanction from the NTSB will do and basta?

I haven't done any research into the ATC regulations concerning culpability of actions. In the US, almost anyone can "be sued". The NTSB (as far as I know) is not a sanctioning body. It would perform an investigation, and submit a report, then some other body (FAA?) would take some action. I would imagine the ATC is already looking into the event and if two controllers put the aircraft on the same path and altitude ( I don't believe this is true ) there will be ( at a minimum ) training or retraining of controllers.

I know this because the authors description corresponds to severe turbulence according to the guide I linked. Actually, upon further inspection, it wasn't even as bad as severe turbulence. Severe (usually) ends up with injuries to passengers not seatbelted in. http://www.aviationlawmonitor.com/2014/05/articles/airlines/us-airways-flight-735-airlines-obligation-to-compensate-injured-passengers/ talks about lawsuits. Since the author is a US citizen, traveling domestically, he cannot sue, without proving negligence.

Lastly, there are yet to be published any real, official, findings by anyone. There is nothing in ASRS (I looked). Until and unless there are published findings, proving anything will be difficult. The author SAYS the oncoming plane was at a FL used by east-bound aircraft, but no official findings corroborate that claim, yet.

@articuno, as I stated in my answer, I'm not a lawyer, it wasn't legal advice. I'm not going to get drawn into a series of comments or answers that can turn it into such. As soon as the question was put on hold, I should have stopped commenting. My bad, and it won't happen again.


text:
Do airliners have an "anti-pilot" warning system?
In February 2014 a co-pilot hijacked Ethopian Airlines flight 702 and took it to Switzerland.
Now in March there is some speculation that Malaysia Airlines flight 370 may have been hijacked and destroyed by the pilots - maybe they took a nose dive into the Andaman Sea?
So my question is this: is there an automatic or say anti-pilot warning system on commercial airliners?
In other words, a system that is non-maskable (can't be disabled by the pilot) and which will automatically warn ATC about unexpected conditions (like a sudden decrease in altitude)?

I'm not sure 'anti-pilot' is the phrase you're looking for here, but I don't know what else to suggest.

The system you're asking about is called radar: ATC should monitor it to see what the plane is doing and ask what is going on when something unexpected happens.
It's hard to "automate" these kinds of alerts. For example, a sudden dive can also be cause by an uncontrolled control-surface hard-over (like the boeing 737 rudder problems that brought down 2 planes), or a loss of cabin pressurization (which requires a rapid descent to an altitude where your passengers can breathe).
The other option is having a second person in the cockpit who can wrest control away from the disobedient pilot (or let people in the cockpit to help).

Does a radar show altitude?

@AdityaPatil Yes, secondary radar shows altitude. But the radar transponder can be switched of by the pilot and then there is no altitude visible. Also, over sea / ocean there is often no radar coverage.

A dive can also be because of a depressurization.

some primary radars can provide altitude (by looking at the angle and range) but that is inaccurate

@lnafziger yes, but in tht case, pilot would give a distress signal.In case of MH370, there was no such signal.

Anything other than radar that pilot can't disable?

@AdityaPatil; there's not such thing as "can't be disabled" as soon as it's aboard the aircraft.

@AdityaPatil if it is electronic then the pilot can simply pull the fuse

@AdityaPatil and what would it do anyway? Utterly useless, unless you're hoping for some system where you can remotely take control over the aircraft at any time. And a hacker/terrorist would just love that capability.

@AdityaPatil - I think you're missing a more major point.  We don't want to put a pilot in a situation where a computer forces control away from them.  Computers can't be programmed for every contingency and they can't think creatively, so it could be dangerous.  Rather, airlines do their best to screen pilots.  A pilot of a 777 has to have several thousand hours of flight time before they are allowed to fly that plane, they are screened throughout the process of gaining hours.  That's how this issue is dealt with, not by letting the computer steal control.

@DeltaLima Secondary radar only tells ATC the reading of the plane's altimeter: it doesn't independently measure the plane's altitude. There was a crash (sorry, can't remember which but there was a Mayday episode about it) in which the flight crew were convinced that their faulty altimeters were correct because they agreed with the figures that ATC was giving them over the radio: neither side realised that the ATC display was just a parroting the incorrect altimeter reading.

@DavidRicherby I think it was Aeroperu 603.

@reirab I think you're right, yes.


text:
Are the current seat belts used in commercial airlines safe enough?
I find myself thinking often, if the seatbelts in airlines are designed just to keep the passengers in seat or to really act as a safety belt like in cars, because in cars which go at 300kmph speed, you have the 5 point seat belt. Why isn't similar approach used for airlines, where the risk is too high.
Plus articles like these, suggest heavier / obese passengers are at more risk.

This is not a duplicate of the existing seatbelt question: That questionis about the locking mechanism while this one is about the choice between 2/3/4/5-point harness.

As a side note, heaver/obese people are more at risk for everything. (Discussion for a different SE, however.)

@FreeMan: Except for starving after a successful emergency landing in a remote area …

For a proper answer we first need to define what "safe enough" is. If you want a significant reduction in risk over current standards, improving the restraint will not be enough.
Your observation is absolutely correct: The lap belt is the minimum the airlines get away with, and you would be much better protected in some kinds of accident by a proper 5-point harness. But fitting the harness alone will not achieve much: To stay in place in a crash, all seats need to be reinforced, so they can tolerate the increased loads at the shoulder belt attachment points. This will add weight and cut into the profits of airlines, so it will only happen when it is mandated.
Please read this answer for more details. It concerns GA aircraft, but much applies to airliner seats as well. The better the restraints are, the more deceleration load can be survived. Note that the points in the Eiband diagrams shown in this answer are from tests with additional leg, chest and arm restraints. If all you have is a lap belt, you might probably be able to tolerate only half as many gs.
The better alternative to a 5-point harness would be to turn around all seats. This would help most to reduce injuries and improve chances of survival in a crash landing.
Now I have to contradict @FreeMan: While the brace position will put all the load on a small strip around a person's hips and lets the arms flail forward, a properly secured torso can take 10g for a duration of more than a second and up to 40g momentarily without ripping the head off. See below the famous sequence of Dr. John Stapp in a ride on his rocket sled "Sonic wind":

Dr. John Stapp in a deceleration test (picture source)
He clearly had his head bobbing about, but even though he lost dental fillings and suffered hemorrhage in his eyes, the head stayed intact.
In the end, all what the lap belt is really good for is to keep you in your seat with negative acceleration due to gusts. This is by far the most common situation when a restraint is needed. The classical brace position (torso over legs, head down) is impossible anyway in economy class due to the ever reducing seat pitch.

Long ago, I was told by a Boeing engineer that the weak point is not the seatbelt - it's the seat. In high-energy crashes the seats often break off the rails and slide around; if the seats were more tightly secured, then people would get wounded by the lap belt.

@egid: Sounds credible. Today's seats should match the strength of the belt, so either of them could fail first.

Great explanation.. By the way the images are some what disturbing. I am so amazed, that most of the times measures for safety are often traded off for cost reasons.The seat turning idea is a great suggestion imo.

Heads rarely snap off, @PeterKämpf, however basilar skull fractures have killed many people while still leaving them eligible for open casket funerals. I agree, it's not guaranteed to happen. It may be that the type of commercial AC crash that is survivable will not include forces that high, while those that do kill people other ways, too, especially since the seats would snap off long before 40G or even 25G.

@FreeMan: How much G's can the standard seat belt bear before snapping? Googled it but couldn't find it.

@PeterKämpf: Your suggestion on backward seats and mins answer, shows that attendant seats have much more safety than the passengers

@Firee: Yes, this is very true. Being fixed to a bulkhead increases safety of those seats even more.

@egid - I've seen similar; the theory is that the most survivable crash is a "crash landing", either with gear up or at too great a rate of descent. In such an event, the gear are designed to snap off (otherwise they'd cause damage to fuel lines risking a fire), and the cargo hold is usually full, so the fuselage doesn't provide much absorption of the impact forces. Really the only thing keeping the passengers from a pelvic or spinal fracture is the spindly legs of the seat crumpling.

The problem, though, is leg fractures; the collapsing seats transfer a lot of load onto the lower leg, and can also trap the foot. The injury in itself is more survivable, but it still means you're going to have to be helped off the plane.

@KeithS: Is there any study on the condition of seats after crashes, it would be interesting to see the findings

Well, for cost reasons, they don't do many crash tests of a complete airplane. One of the best recent attempts I know of is here: https://www.youtube.com/watch?v=WvbGiuKbmGM (this is the BBC version; Discovery ran the same show with VO and editing changes). The MythBusters also did some more backyard-ish tests of airline seats that showed a similar collapse under an impact load: https://www.youtube.com/watch?v=lO9s8Iz-hII (the closest drop to FAA testing standards is at about 24 minutes in).

Rear facing seats would be safer in an impact but some people waves hand get considerable motion sickness riding backwards. Facing forward is not an issue in the least for me. Yet another trade-off in the safety/price/convenience/comfort debate

@Firee Everything is a trade-off between safety and cost, or safety and convenience. To a first-order approximation, the number of fatalities in commercial aviation per year is zero. It just isn't worth spending money to make that number even closer to zero.

Are the current seat belts used in commercial airlines safe enough?
Not really, the current seat belt standard is aimed mostly to protect against aircraft movements encountered in flight, not during impact which are very uncommon, contrary to automotive industry.
Current FAA regulations impose the seat and seat restraints to protect from 16g deceleration using a complex test scenario and simpler 9g test (only the latter before 2009). Some seat belts are fitted with an airbag that expands forwards.
Why?
With a passenger not keen on taking time to understand and adjust a harness, and not willing to remain restrained by a harness for a long time, the compromise is to have a simple belt for flight turbulence, and use the brace position against impact, albeit:

This combination has been proven effective mostly in frontal collision (e.g. in the Hudson ditching).
Lap belt have shown vertebrae problems in car accidents due to the rapid deceleration of the lumbar region:

(Source).

Wikipedia article on seat belt:

Evidence of the potential of lap belts to cause separation of the
  lumbar vertebrae and the sometimes associated paralysis, or "seat belt
  syndrome", led to progressive revision of passenger safety regulations
  in nearly all developed countries to require 3-point belts first in
  all outboard seating positions and eventually in all seating positions
  in passenger vehicles. [...] Besides regulatory changes, "seat belt
  syndrome" has led to tremendous liability for vehicle manufacturers.

Three-point diagonal seat belts, which don't have this problem, are used for helicopters and light aircraft, but are not seen in airliners because the upper anchoring point is not easily available.
Better protection used in commercial aviation
Torso restraints are mandatory for the attendants in the US, but attendants are trained to make them effective and are expected to take the time to adjust them correctly so that actual additional benefits over the lap belt can be obtained.

AC No. 25.785-1B:

In accordance with § 121.311(g), each seat occupied by a flight
  attendant required by § 121.391(a) must have torso restraint (a
  combined pelvic restraint and double strap upper torso restraint with
  a single point release) that meets the requirements of § 25.785,
  except that any combined safety belt and shoulder harness approved and
  installed before March 6, 1980, may continue to be used.

also:

Passenger seats occupied by required flight attendants must fully
  comply with § 25.785(h), including the combined pelvic restraint and
  upper torso restraint.

@mins : Even if we don't consider a five point harness, wouldn't a simple seat-belt like approach, as in cars, would provide atleast some support to the torso?

@Firee, yes, they would. The seats, however, would have to be redesigned to be strong enough for the upper mount point to be useful. This can be done (many car seats have the belt anchored to the top of the seat), but it adds weight. Fixed weight in the air frame reduces cargo carrying capacity and increases fuel costs. It then becomes a trade off to see if/how often the added protection of the shoulder harness would benefit a passenger in a "typical" crash scenario. For in-flight safety in turbulent conditions, the lap belt is sufficient.

@FreeMan: I was reading this about harness use: "Accident experience has provided substantial evidence that the use of a shoulder harness in conjunction with a safety belt can reduce serious injuries to the head, neck, and upper torso of aircraft occupants and has the potential to reduce fatalities of occupants involved in an otherwise survivable accident." Source: Transport Canada AC 605-004

I would postulate (but have no firm evidence to prove the point) that the brace position is actually safer than a 5-point harness would be for passengers in an aircraft accident.

The 5-point harness would hold your body snugly against the seat, but leave your head bobbing around as a 5-pound ball on a weak tether (your neck). The same issue would happen to you in your family car with a 3-point harness, and that's one of the reasons it has airbags installed.
The brace position has your head fairly firmly planted in the furthest forward position already, so your head doesn't whip forward during that sudden stop.

Why do race drivers wear a 5- or 6-point harness then? They also wear a HANS device which effectively "seat belts" their head to the car, as well. The HANS device leaves their head with 1-2 inches of  forward movement and absorbs the stress of impact* with tethers designed for it.
*One recent F1 accident saw impact forces upwards of 25G. The driver unbuckled himself and got out of the car while the track marshals were still on their way. I'm sure he was sore, but was really no worse for the wear.

I would add that pilots do have 5-point harness, but the primary reason is that they need it for support in turbulence. Without the harness even moderate accelerations could cause inadvertent control inputs.

Note, I did indicate passengers in my answer. Your point about the pilots is quite valid. Of course, they have the same bobble-head problem the passengers do, but if the stop at the end of the flight is sudden enough, the pilots usually don't make it anyway.

As a F1 fan myself, i agree with the Hans device implementation a few years ago, it helps drivers withstand upto 30-40G impacts.

Counter to the points about HANS - part of the need for it is the significant additional weight of the crash-helmet that drivers wear.

Yes, the helmet does add additional weight, @MikeB, however, your head is heavy enough on its own that a sudden stop will most likely cause notable damage to your neck without a helmet and could, potentially, break your neck.

The short answer is that current airline seats and seatbelt systems are pretty safe when used in accordance with their design criteria. They are not merely decorative - they are tested and proven safety devices.

The longer answer requires looking at the regulations and understanding what the seat/seatbelt/aircraft interior design as an overall system is intended to do for passengers in the event of an incident.
The current seat belts on FAA-certificated transport-category (airliner) aircraft are governed by FAR 25.785.
I've excerpted a few relevant bits of that regulation below.

Each seat, berth, safety belt, harness, and adjacent part of the airplane at each station designated as occupiable during takeoff and landing must be designed so that a person making proper use of these facilities will not suffer serious injury in an emergency landing as a result of the inertia forces specified in §§25.561 and 25.562.

Straightforward enough: "If we crash people sitting in the seats shouldn't suffer serious injury."
The loads the seats and seat belts must withstand are, in part, described in FAR 25.561 and 25.562 which deal with emergency landing conditions - among the requirements are for the seat and seat belt to handle a 9 G forward load with FAA-Standard 170-pound adults.
The 170-pound adult certification criteria is why you will find discussions about how larger/heavier individuals are "less safe" -- a heavier individual will impose more load on the seats, seat belts, and attachment points, and may exceed the ultimate limit load of an approved seat/seat belt design.

Each occupant of a seat … must be protected from head injury by a safety belt and an energy absorbing rest that will support the arms, shoulders, head, and spine, or by a safety belt and shoulder harness that will prevent the head from contacting any injurious object.

In plain English:
 "Either you need to be secured with a shoulder harness so you can't hit anything, or the seat in front of you needs to be soft and padded so it won't crack your skull open and kill you when you're in the brace position."
(This is the same logic used in school busses: Rather than strapping every passenger down with a 5-point restraint we make sure they'll stay in their seat and everything they can hit from a seated position is soft enough that it probably won't kill them.)

Each seat or berth, and its supporting structure, and each safety belt or harness and its anchorage must be designed for an occupant weight of 170 pounds, considering the maximum load factors, inertia forces, and reactions among the occupant, seat, safety belt, and harness for each relevant flight and ground load condition

i.e. "In a worst-case crash a FAA-Standard 170 pound adult shouldn't break the seat or rip the seat belt out of the attachment points." as I described earlier.  In practice the regulation sets a minimum standard, and most seats will withstand more force than that.

Sadly, in America, at least, there aren't too many FAA-Standard 170 lb passengers. I wasn't 170lbs as an average height, healthy, active college student, and it's gone downhill since.

To add to @FreeMan's concerns, the average or taller person will physically not be able to bend down to the brace position. When I sit in a standard economy seat with 31" pitch and bow down, my face will hit the seat in front just above the foldable tablet (which isn't soft by any measure). It will not go any further unless I get some more joints in my spine.

@FreeMan One day the certification standards will be updated (the "standard passenger weight" in AC 120-27E is 190 pounds now), but that day is not today :)

Aircrafts' seatbelts are for security reason. And yes, security while flying is "just to keep the passengers in seat" in case of a void or, on ground, in case of a bump or the plane leaning.
Apart from that cases, there is no need for a complicate 2/5/10k point system in an impact against a mountain: you will be just plain dead anyway.

Security? Usually, if I'm not mistaken, "security" in English means protection against intentional harm, whereas "safety" is protection against happenstance.

@HenningMakholm: thanks for your feedback. Feel free to use the Edit button to correct the answer, then :-)

There is something in between a slow graceful stop and hitting a mountain. Rapid deceleration would clearly benefit from more than a 2-point harness.


text:
Why do small aircraft rentals charge per flight hour?
Why do airplane rental companies charge per flight hour (using hobbs time), instead of either MX hours (tach time) or based on the amount of fuel used?
Wouldn't a 3-hour long cross-country flight be much cheaper than three hours spent in pattern work?
In my opinion, rental rates would seem more logical if they are based on MX time.

cause motor and general maintenance and fuel are time based rather than flight based

I think @Sponge Bob should have asked Hobbs versus tach?

@newmanth That is what I meant to ask.  Tach=MX time and flight time=hobbs.  Is that not correct?

@SpongeBob yes, that's correct. a hobbs is optional, but even the lowly 40-year-old Citabria I used to fly had tach time

Somewhat discussed here and it indeed seems tach time would be better, because hobbs might encourage pilots to cut run-up short to save money.

Similar Question: What is the difference between Hobbs Time and Tach Time?

In the US most General Aviation rentals are "Wet Hobbs Time" -- You are billed based on the actual clock time the engine was running (in tenths of an hour), and fuel/oil are included in the rental price.
This is attractive to renters because of the simplicity: You don't have to pay the FBO separately for the fuel, oil, and aircraft, and you're basically paying for the amount of "clock time" you spent in the plane.
It's easy to understand, especially for new students who may not quite grasp the concept of how the recording tachometer measures "time".
Hobbs billing is attractive to the FBO as well, for different reasons:

Rentals are typically blocked out by clock hours ("10 AM to 2 PM").
Since booking is clock time billing based on clock time (Hobbs hours) makes sense.
In practice the two numbers are usually close (and when they're not the error tends to favor the FBO).

Let's look at your "Tach vs. Hobbs" scenarios (for the sake of simplicity let's assume both flights are three "clock hours" chock-to-chock):
Pattern Work
Pattern work consists of three basic phases:

Takeoff (full throttle, high RPM - Tach runs faster than Hobbs)
Circuit (low power, low RPM - Tach runs slower than Hobbs)
Descent & Landing (near idle power, very low RPM - Tach runs MUCH slower than Hobbs)
There might be a taxi in there for full-stop practice, but you'll be close to idle there too.

If we average this out over three hours of pattern work (and assuming you and your instructor don't commit suicide to break the monotony) the Tach hour meter will be reading a number much lower than the Hobbs meter.
In this scenario you might have 2.25 tach hours for your 3 Hobbs / Clock hours.
3-hour Cross Country
A cross country consists of 5 phases:

Takeoff/Climb (full throttle, high RPM - Tach runs faster than Hobbs)
Cruise (moderate throttle, moderate RPM - Tach and Hobbs run about the same speed)
Descent (moderate-low throttle, moderate-low RPM - Tach may be a bit slower than Hobbs)
Circuit/Approach (low power, low RPM - Tach runs slower than Hobbs)
Landing (near idle power, very low RPM - Tach runs MUCH slower than Hobbs)

If we assume Phases 1 and 3 take about the same time they roughly cancel each other out. Phase 2 (Cruise) is a wash too, so we're left with two brief periods where the tach is running noticeably slower than the Hobbs (the pattern and landing at the destination).
In this scenario you might have 2.85 Tach Hours for your 3 Hobbs / Clock hours.

In both of these scenarios the FBO comes out ahead of the game -- they're billing you for 3 hours of time, but their maintenance cycle is based on the tach. This might be enough to squeeze another day or two of profitable flying out of an airframe before it's next annual or 100-hour inspection.

There are scenarios where you can "beat" the Tach clock, but most of them involve being downright abusive to the aircraft/engine. For example if you flew that 3 hour cross country at low altitude and full throttle (very high RPM) you might rack up 3.3 tach hours for your 3 Hobbs hours. The FBO is basically stuck eating that maintenance expense.
Along similar lines when renter pilots abuse an engine (for example by leaving the mixture knob full rich for the whole flight) the tach time doesn't always tell the whole maintenance story - the mechanic's time cleaning the plugs because the next renter couldn't get through a runup has to be paid for somehow.

Note that it's also possible to do pattern work and have the tach time equal to or slightly ahead of the hobbs time -- it all depends what portion of your flying is spent in the takeoff/climb phase. I did much my training in an anemic Cherokee 140, and my instructor was not a small man. The two of us and full fuel put us within a few pounds of gross weight, and on warm days climbing to pattern altitude for another circuit could take quite some time!

The fuel-to-air mixture in aircraft piston engines is mostly set with manual controls. A lean mixture will result in higher cylinder head temperatures and increase the risk of detonation, preignition and overheating of exhaust valves. This means that a pilot who is charged according to fuel consumption will use a different mixture setting than a pilot who is charged by the hour. To conserve fuel, the mixture can be set leaner than what would be best for preserving the engine.
Basically, piston aircraft are charged by the hour to generate the right incentives for setting the fuel-to-air mixture. Also, as ratchet freak mentions correctly in the comments, the secondary cost are mostly proportional to operating time.

You don't mention hobbs time vs. tach time however. And the premise of the question is that hobbs time is used more often though tach time should generate better incentives.

Are you not already charged based on fuel consumption by having to top up the tanks when returning the plane, or is that only when renting a car?  (IANAP - I am not a pilot)

@FreeMan A common arrangement is for airplanes to rent “wet,” that is, inclusive of fuel costs.

@JanHudec: Yes, I only focused on the aspect of charging based on fuel used, the second question. But I don't see much of a difference in hobbs and tach time between a cross-country flight and pattern work. After all, flight speed is lower in pattern work and so should be tach time. I think the premise of the first question is wrong.

The FBOs make a real profit when they get someone who sets a leisurely 65% cruise power and leans properly :)

Also I always saw wet rentals (no matter how you charge) as a disincentive to set the mixture properly: What does the renter care how much fuel they're throwing away or how dirty the plugs get? It's paid for anyway!

@GregBacon thanks, interesting info. I guess you can get a car rental that way, too. Guaranteed price to fill up the tank.


text:
Why would the Boeing 777 not be included in Jane's All the World's Aircraft – In Service?
I am currently sitting in front of the last nine issues of Jane's All the World's Aircraft – In Service at the British Library (issues 2013–2014 to 2023–2024).
To my great surprise, the Boeing 777 is not included in any of these issues.
Am I missing something obvious?
Just to make sure, I even checked under "T" (for "Triple-Seven")…, but still nothing. At a price tag of USD ~2,000/volume, I would have expected Jane's not to omit an aircraft like the 777 from their volumes.
Edit: The 787 Dreamliner is also missing from the latest issues…. Could it be that Boeing is not keen on Jane publishing a nicely formatted list of aircraft weights and performance parameters?
For example, here is the index of the 2013–2014 issue:

Interestingly, the 777 can be found in the online version.

The Next Gen series of 737s is also not shown on that list (737-600 thru 737-900). Is there another list of Boeing aircraft under something besides that “Boeing Commercial Airplanes” header?

nor is the 747-8. Neither are the 767-300 and -400 passenger versions

You're looking in the wrong volume. The In Service volume contains aircraft types that are in service but out of production. You need to look in the Development and Production volume, as the 777 is both in production and has variants under development.

Wow - that is some really bad choice of Title !


text:
What is the validity duration of FAA Medical for international flights?
FAA Rule:
The duration of FAA medical certificates is specified in 14 CFR 61.23.
An example would be for:

A first-class medical certificate
Applicant is under age 40
Applicant is an airline transport pilot certificate for pilot-in-command privileges

In this case:

Then your medical certificate expires, for that operation, at the end of the last day of the 12th month after the month of the date of examination shown on the medical certificate.

So this clearly states that if I pass my medical on January 1st 2014 then my medical is good until the end of the day on January 31st 2015.
ICAO Rule:
However, I have been told that ICAO rules state that the medical expires 12 months to the day from when it was issued, which would mean that it would no longer be valid as of January 1st, 2015.  (A reference for this would be great.)
Question:
As the holder of an FAA pilot license, with the above FAA issued medical, may I legally fly outside of the US after January 1st 2015 (but before February 1st 2015)?

I believe the ICAO regulation you are thinking of is this one:

Excerpts from ICAO Annex 1- Personnel Licensing
“1.2.5.2 Except as provided in 1.2.5.2.1, 1.2.5.2.2, 1.2.5.2.3,
  1.2.5.2.4 and 1.2.5.2.5, a Medical Assessment issued in accordance with 1.2.4.5 and 1.2.4.6 shall be valid from the date of the medical
  examination for a period not greater than:
60 months for the private pilot licence — aeroplane;  12 months for
  the commercial pilot licence — aeroplane;  12 months for the airline
  transport pilot licence — aeroplane;”
It also indicates the following in a note:
“Note 2 — When calculated in accordance with 1.2.5.2 and its
  sub-paragraphs, the period of validity will, for the last month
  counted, include the day that has the same calendar number as the date
  of the medical examination or, if that month has no day with that
  number, the last day of that month.”
In ICAO Annex 1, 2.4.1.6, the medical requirement for a commercial
  pilot is prescribed. It states:
“2.4.1.6 Medical fitness
The applicant shall hold a current Class 1 Medical Assessment.”
In ICAO Annex 1, 2.5.1.6, the medical requirement for an airline
  transport pilot is prescribed. It states:
“2.5.1.6 Medical fitness
The applicant shall hold a current Class 1 Medical Assessment.”
“6.1.1 Classes of Medical Assessment
Three classes of Medical Assessment shall be established as follows:
a) Class 1 Medical Assessment; applies to applicants for, and holders
  of:  — commercial pilot licences — aeroplane and helicopter  — airline
  transport pilot licences — aeroplane and helicopter  — flight
  navigator licences  — flight engineer licences
b) Class 2 Medical Assessment; applies to applicants for, and holders
  of:  — private pilot licences — aeroplane and helicopter  — glider
  pilot licences  — free balloon pilot licences
c) Class 3 Medical Assessment; applies to applicants for, and holders
  of:  — air traffic controller licences.”

However, ICAO is not involved in enforcement but rather coming up with standards, which would then be enforced (if adopted) by the member country's aviation authority.
From the FAA:

There are many differences to be aware of when flying outside the
  United States (U.S.). International Civil Aviation Organization (ICAO)
  rules prevail. Member states follow ICAO guidelines as published, with
  differences noted, in their official Aeronautical Information
  Publication (AIP).
  http://www.faa.gov/air_traffic/publications/ifim/intl_overview/

For example, the EASA (European Aviation Safety Agency) has rules very similar to the FAA:

MED.A.055  Validity, revalidation and renewal of medical certificates
  (a)  Validity  (1)
  Class 1 medical certificates shall be valid for a period of 12 months.
  (2)
  Notwithstanding to paragraph (1) the period of validity shall be reduced to 6 months for pilots
  who:  (i)
  are engaged in singlepilot commercial air transport operations carrying passengers and
  have reached the age of 40  (ii)  have reached the age of 60
  (http://easa.europa.eu/rulemaking/docs/npa/2008/2008-17/NPA%202008-17c.pdf)

However, according to this report: http://dspace.mit.edu/bitstream/handle/1721.1/35725/FlightOpsICATfinal2.pdf,

Air carrier operations are generally regulated by the country of
  registration and the sovereignty in which the operation is conducted,
  including the Federal Aviation Administration (FAA) in the USA and the
  Joint Aviation Authority (JAA) in Europe. International flights may
  also fall under the jurisdiction of ICAO (International Civil Aviation
  Organization) when operating abroad (U.S.-based carriers are governed
  by both FAA  and ICAO regulations when conducting international
  operations.)

So you might be legal until you hit the international waters.  At the very most you might be able to go abroad, but then not be legal to fly back.
The National Business Aviation Association (NBAA) recommends that pilots comply with the strictest interpretation of the law (so in this case, have the appropriate compliance with ICAO standards) to avoid having to explain the nuances between the two sets of regulations.
(http://www.nbaa.org/admin/personnel/medical/medical-certificates.php)


text:
How does ICAO differentiate between Commercial and Private flights?
How is the difference between Commercial and Private/General Aviation flights defined by ICAO?
Here in the US, we use 91 flights -vs- 135 flights, but from what I've heard they use a different definition, particularly concerning business aviation flights that fall under Part 91.
In other words, what determines whether ICAO Annex 6 Part 1 or Part 2 applies to a particular flight?

According to ICAO Annex 6 Part 1 (PDF):

The Standards and Recommended Practices contained in Annex 6, Part I,
  shall be applicable to the operation of aeroplanes by operators
  authorized to conduct international commercial air transport
  operations.

According to ICAO Annex 6 Part 2 (PDF):

International general aviation operations with:
a) aeroplanes with a maximum certificated take-off mass exceeding
  5,700 kg; or
b) aeroplanes equipped with one or more turbojets engines.

The International Flight Plan form also requires flight type to be declared, but I am not sure that this would be ICAO's sole requirement.
Type of flight—Indicate which one:

S—Scheduled air transport
N—Non-scheduled air transport
G—General aviation
M—Military
X—Other

For a simpler explanation, this PDF lists civil aviation activities:

This page contains a table (at the bottom) classifying each type.

Okay, so what is the difference between "commercial air transport operations" and "general aviation operations"?  That's what I'm trying to figure out. :-)

@Lnafziger "Commercial" means you are engaged in commerce, i.e. basically, money is changing hands. "Air transport" means you are transporting something (goods or people). On that basis alone the ICAO text could be read as "if you're moving stuff around for money, you're subject to Part I." Not sure if they offer a more precise definition or not.


text:
Which countries allow pilots to fly over the age of 65 for International Part 135 Operations?
The FAA does not have an age limit for pilots flying under Part 135 regulations, however ICAO specifies an age limit of 65 (with other restrictions after the age of 60).
The NBAA has a nice Age 60 Briefing which discusses this, but at the end they say:

Most of the States that have authorized their pilots to fly as
  pilot-in-command in commercial air transport operations after they
  reach the age specified in 2.1.10.1 also authorize pilots holding a
  license issued or validated by another States to fly in their own
  airspace under the same condition.
However, ICAO does not collect information on States authorizing
  pilots to fly in their airspace after reaching the age of 60 and
  cannot provide information on the subject. Pilots seeking such
  information must contact individual Civil Aviation Authorities.

Is there any resource available which covers which countries allow pilots to fly over the age of 65?  For instance, I know that Europe does not allow it, but that some others do.  I am particularly curious the countries close to the US, such as Canada, Mexico, the Bahamas, and those in the Caribbean, Central and South America.

Getting up there, are we? :)

@CGCampbell Lol, actually no.  But I want to know what my options are if I hire someone who is.  ;-)

Are you looking for a list? Or what happens to a 70 year old flying into a country with the age 65 limit?

@ben I'm looking for a list or resource that can be used to determine if a particular country applies the age 60 rule, thanks.

Australia, Austria, Brazil, Canada, Costa Rica, New Zealand, Russian Federation, Senegal, and Ukraine
According to a 2006 ICAO report (mentioned also in this presentation) (emphasis mine),

Nine States (Australia, Austria, Brazil, Canada, Costa Rica, New Zealand, Russian Federation, Senegal, and Ukraine) have no upper age limit, whereas fifty-five have an upper age limit higher than 60 years, ranging from 62 to 72 years.

This, of course, applies only to domestic flights, not international flights, as I imagine you have already read about:

The ICAO Council adopted on 10 March 2006 an amendment to Annex 1 — Personnel Licensing that increases by five years the upper age limit for commercial pilots operating two-pilot aircraft, subject to conditions. The new provisions become applicable on 23 November 2006 and read as follows:
2.1.10.1 A Contracting State, having issued pilot licences, shall not permit the holders thereof to act as pilot-in-command of an aircraft engaged in international commercial air transport operations if the licence holders have attained their 60th birthday or, in the case of operations with more than one pilot where the other pilot is younger than 60 years of age, their 65th birthday.
2.1.10.2 Recommendation.— A Contracting State, having issued pilot licences, should not permit the holders thereof to act as co-pilot of an aircraft engaged in international commercial air transport operations if the licence holders have attained their 65th birthday.

Note that the ICAO typically gives constituents recommendations, not necessarily enforceable laws. Those are generally up to the countries themselves, which is why these nine are able to not have upper age limits.
I wanted some confirmation on the original list from something more recent, so I checked out a few sources. I was only able to find information for two.
Australia
From the Civil Aviation Safety Authority, it is noted that the ICAO age limits for pilot in command (PIC) and co-pilot are not relevant in Australian domestic flights, and seems to imply that age does not affect the ability to hold a pilot's license.
Canada
The best I could find was this article from The Globe and Mail, originally from 2009 but possibly updated in 2012.

There is no upper age limit to acquire or renew a pilot licence in Canada - all you have to do is log the hours, complete the training and pass the medical exams, when necessary.

This is good information, but covers only a small part of what I am looking for.

Quote from the source:

Under international aviation regulations, there must be at least one pilot under 60 in the cockpit at all times, and pilots over 65 cannot hold the captain position.

Specifically for Canada:

A new collective agreement, imposed by an arbitrator, sets out specific positions that pilots over 60 and over 65 can hold on Air Canada’s aircraft.

From the sources Here and Here, its safe to say that the maximum age of 60 is applicable for some operations and for some it is 65. (For any airline operations to United States)

Hi Vivian, thanks for the answer, but that ruling only covers the US side of things, (and doesn't even apply to most 135 operations because of the type of aircraft that it applies to).  I'm specifically looking for which countries have this rule and which ones don't though.

It applies to all of the "operations to US" from the aforementioned countries, and a fore mentioned countries, and operations to and fro from canada

Your second link explains that it only applies to scheduled operations and to non-scheduled operations on aircraft with more than 30 seats or greater than 7,500 lb. payload capacity.  What about outside of those situations (i.e. non-scheduled operations on 12 seat aircraft with only a 5,000 payload capacity)?

Edit the question specifically to a given operation dude :p

Well, it is a general question and you only answered part of it is what I am trying to point out...  Actually, in reference to your last sentence, you took your source out of context and make it sound like that always applies when it doesn't.

Laws of international aviation ensure for multi crew operations that one cant be above 65 nd be a pilot simultaneously

@VictorJuliet There's not really any such thing as "laws of international aviation." ICAO issues recommendations which member states may or may not adopt as actual laws. As Lnafziger pointed out, your last 2 links (the one from the FARs and the one from the FAA) specifically say that the FAA only enforces those age limits on non-scheduled ops if the aircraft is configured with > 30 passenger seats and/or has a payload capacity > 7,500 lb.


text:
Can I log time in a Cri-Cri twin-engine aircraft?
I was looking at potential experimental projects when I read this fascinating website about a tiny aerobatic-capable twin-engine airplane. It's light enough to be an ultralight, but much too fast:

Aside from the obvious fun of flying this little plane, I wondered whether:

I'd be able to log time in the Cri-Cri as multi-time? My guess is yes.
Assuming I'm MEL-IFR, could I log multi-IFR with a two-way radio, altimeter, Dynon-type AI, HI and at least one cert. VOR & glide slope? An approach cert. GPS setup would be too heavy I assume. My guess is this is wishful thinking...

That would not be a fun ride in hard IFR.  Climbing through a layer would probably be OK, but anything more just sounds uncomfortable for something so light.

There is also a version of the Cri Cri with 2 turbine engines. Pop in an oxygen system and drive the airlliners crazy!

What a positively delightful looking aircraft!
I see no reason you couldn't log time in this as multi-engine time - it clearly has two engines, with two separate sets of controls (at least throttles, from what I can see).
Similarly I see no reason you couldn't fly it IFR as an experimental aircraft, provided it's "properly equipped" (consult the FARs for all the details on what you need), passes an IFR pitot/static test, and has appropriate radios (Transponder which you'd likely need anyway, Com, and Nav).
Whether you'd want to take it up in real weather is another matter though :-)
For that matter if you want GPS you could certainly have it without much weight penalty: throw in a Garmin GNS 430W and cover the Nav/Com and GPS in one ~7-8lb box.  Whether or not that kind of investment in what's basically a single-seat super-ultralight is "worth it" is a judgment call only you could make, but it definitely seems possible.

Never mind the weight... where the heck you gonna stash that 430W?! Also, just a nitpick, IFR certification doesn't care about pitot. It's just a static system test.

Yep, as long as your avionics are certified and you follow all the rules for experimental aircraft in IFR you can log it. It's questionable whether or not anyone would accept the time when you go to get a job, but they might.
I would imagine that you're right about the avionics weight, although glass might actually reduce the weight.

I suspect most hiring managers would argue that Cri-cri time does not equal King Air time

Yes, but I'd get points for creative thinking...

Who cares. At that point you have an MEI and ATP in your pocket with the world's cheapest twin time.


text:
For airplane spotting, what minimum specs should a telescope have?
We are thinking about buying a telescope for viewing planets, stars, the moon, and aircraft! What is the absolute minimum zoom and/or size to achieve getting a good glimpse of an aircraft at its cruising altitude?
Does anyone have experience doing this that they could share to help us out?

I'm pretty sure you can watch aircraft if you can watch planets, as you get a pretty good glimpse with the naked eye or with a pair of binoculars :) (cruise altitude is ~30k ft) The bigger problem will probably be keeping it in sight (moving at ~ 400kts), and actually seeing the whole aircraft rather than the fly splattered on the fuselage...

I'm with @falstro, a telescope that's good for astronomy is going to be a bit strong for aircraft.  Perhaps a good pair of binoculars would be better? Especially with how much a plane can move...

do keep in mind you'll see planes upside down when using an astronomical telescope.

@shortstheory or watching an aerobatic contest :-)

You might want to use the term "magnification" rather than "zoom" (which means something different in photography at least)

I've had fantastic airliner views with a 6" f/6 alt/azm mounted reflector. Don't pick up a gem style mount, and you want something with a relatively wide field of view. Orion Starblast 6 would fit the bill, and is a decent starting scope for astronomy.

I would not recommend a telescope for viewing aircraft.
As others have pointed out, a telescope that is suitable for astronomy would be incredibly overpowered for viewing aircraft - the aircraft would move out of the viewing frame very quickly, and you would have a difficult time tracking them.
I would suggest picking up a spotting scope (the kind of thing bird watchers and hunters use - good ones can be had in the 50 to 100 dollar price range, and excellent ones abound from about $100 up), or a pair of good binoculars for your plane watching. The zoom factor is far better suited to plane spotting, and either option would be substantially more portable than a full telescope rig.
As for advice on a telescope, there is an Astronomy stack exchange site, and while the entire network generally doesn't do "product recommendations" the Astronomy site's chat might be a good place to ask for advice on picking a telescope. (I'm not sure how active their chat room is, but it's worth a shot!)

While Astronomy doesn't do product recommendations we do cover "how to find the right telescope for your requirements". We just don't allow posts which only recommend a specific product without more information.

Others have said that a telescope suitable for astronomy would be overpowered for viewing aircraft, but this isn't true.  Manually keeping aircraft in the field of view would be difficult, but you can use software to automatically steer a computer-controlled telescope and get great results.
For example, here's a photo of an aircraft being tracked by the tracking application OpticTracker:

This demo video shows both a Meade LX200 and a Celestron NexStar 4E being used with OpticTracker to track various aircraft.
There may be other tracking applications out there; OpticTracker is just one I've researched. In order to select an appropriate telescope, I would start with the requirements for whatever tracking software you're considering using.  For example, you can see the list of telescopes compatible with OpticTracker on the compatibility page.

Impressive tracking! Note that it's also perfectly possible with manual tracking, e.g. on a dobsonian.

I've been spotting for years with my astronomical telescope.  I typically use low to medium power to make tracking easier.
The key is to start behind the plane, and raise your scope until you find the con trail. Focus on the con trail, then follow it to the plane! If you lose the aircraft, find the con trail again and follow it.
I'm also now using the flightaware app to cross reference the craft I see.


text:
Can I find out why an aircraft squawked an emergency code (7x00)?
I have recently been using a mobile app to track flights, which is really cool.  I live in the rural heartland of America, so it's an event for me to see an A380 actually flying.
Every once in a while, a squawk 7700 alert will come up, which I understand is the emergency transponder code. There are more, such as 7600 and 7500, which I find are less common.
My question is, is there a way to do some post-mortem followup as to why the aircraft squawked the code?  Is this public information that can be found by some agency such as the NTSB?

The Aviation Herald publishes some of these events. The FAA (and corresponding agencies) publish periodic summaries of these events as well (if they are reported).

That would actually be a really good answer :)

This FAA webpage has a list of the various transponder codes and what they are used for, which will give you an idea of what the problem is.  Unless there is an actual aircraft incident or accident which requires the FAA to investigate though, you won't be able to find a record of it.  In fact, pilots aren't even required to file a report unless requested, and those don't go into a public database.
For actual incidents or accidents, you can look at the NTSB Accident Database & Synopses.
For pilot reports to the NASA ASRS self-reporting safety system, you can search their database.  If the pilot filed a report then you will see it show up there.
Commonly used emergency codes are:

7500 - Hijack Code
7600 - Loss of Radio Communications Code
7700 - Emergency Code

Also, some countries use the code 7000 for VFR traffic (which is not an emergency code), but the US uses 1200 instead.

I have seen a mnemonic for the emergency codes in the past. I do not recall where I saw it, and I don't know how common it is, but I found it helpful in remembering them: "75, taken alive; 76, caught a glitch; 77, going to heaven".

I know "75 - man with knife"

An emergency requiring a Service Difficulty Report (SDR), generated by owner self-reporting or investigation results, is reported by tail number. If you know the number, or other equipment identifying attribute, you can search here.
For reports filed less than a week to 6 months after incident, FAA packages are published within a week. You can find these on this FAA data source.

I am not sure this answer the question. where are the squawk codes?

I think it answers the question. He's saying to find out why, write down the tail number then search some time later.


text:
How does an airliner deal with cabin altitude at an airport above 8000 ft?
There are a number of airports above 8000 ft altitude. FAR 25.841 states:

Pressurized cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet at the maximum operating altitude

How does an airliner operating from a high airport, say from El Dorado, deal with cabin pressure? Do they actually pressurize the cabin after takeoff and depressurize it after landing?
Note: the answer can be of any jurisdiction though FAA seems relevant here as US airlines operate from there.

lol, I would not want to be the one holding on to the cabin door handle to unlock:)

"Do they actually pressurize the cabin after takeoff and depressurize it after landing?" Why not? That's what happens with every other flight.

Here is the text extracted from Boeing's 777 Flight Crew Operations Manual:

In cabin altitude controller cruise mode, maximum cabin altitude is 8,000 feet. When the takeoff field elevation is higher than 8,000 feet, the cabin descends to the cabin cruise altitude while the airplane is climbing.
When the destination airport elevation is greater than 8,000 feet, cabin altitude controller cruise mode maintains a cabin altitude of 8,000 feet.
(...)
At touchdown, the outflow valves open to depressurize the cabin.

Therefore, when the airplane is on the ground, its cabin altitude is always equal to the ambient altitude. When it is in the air, the maximum is 8000 feet. Whether the cabin altitude will climb to 8000 feet or descend to 8000 feet depends on field elevation.
The reverse happens upon landing. If landing at an airport at 10,400 feet, the cabin altitude is <= 8000 feet until touchdown, then raises to 10,400 feet.
This make sense - if the outside pressure is greater, how do you open the door? (well with enough force you can, but I will leave it to your imagination as to what happens the moment the door is opened).

Many doors and flight deck windows are plugs, so if the outside pressure is greater the doors / windows will be forced inwards. As the seals are not designed to work in that direction air will quickly flow in until it equalizes.

As a matter of practical experience, I've flown as a passenger out of El Alto, Bolivia, 13,325 ft. I did not feel there was any change in pressure during e.g. taxiing. To my ears, the climb-out felt just like descending to land.

Riding as airline passenger, I've observed another passenger experience recovery from altitude sickness  (headache, disorientation) during the climb out of La Paz Bolivia.  Simultaneous the pressure altimeter function on my hand- held GPS showed the pressure altitude steadily dropping to about 7000 or 8000 feet.

I am an aerospace engineer, who has worked on the design and certification of pressurized aircraft, so let me try and answer.
The class of regulations that deals with the certification of pressurized aircraft are called 14 CFR 23.841 (General Aviation) and 14 CFR 25.841 (Commercial). Both paragraphs are pretty much identical and in summary say the following (paraphrasing):

All GA and commercial aircraft designed and certified to cruise at
  altitudes greater than 25000 ft must be pressurized.

This means that aircraft whose maximum altitude (often referred to as "service ceiling") is less than 25000 feet do not have to be pressurized (once you exceed 14000 feet, most people begin to use supplemental oxygen, but above 25000 feet your plane must be pressurized and that's what provides your supplemental oxygen).
Unpressurized GA aircraft routinely operate at altitudes up to 25000 feet. This should answer what pressurization an airliner must provide at high-altitude airports: None! Not unless said airport was above 25000 feet would some pressurization be required (a problem that does not exist on this planet).
What paragraphs 23.841 and 25.841 are saying is that those airplanes that are pressurized, must be capable of providing cabin altitude of no more than 8000 feet at their operating altitudes. So consider two aircraft, one operating at 25001 ft and the other at 45000 ft. Both must provide no less than 8000 feet in the cabin. It shouldn't come as a surprise that the one operating at 45000 ft will require a stouter (i.e. stronger and thus heavier) fuselage as the pressure difference between the cabin and outside is much greater. Many modern commercial airliners provide a 5000 feet cabin altitude and some bizjet manufacturers are already considering sea-level pressure in the cabin, but I digress.
Anyway, I hope this sheds some light on this mystery. Cheers.

What is S-L pressure?

@CGCampbell: Sea level pressure.

I'm having trouble understanding how this answers the question at all. The FARs referenced regulates what pressure difference the aircraft must be able to provide; not what the cabin altitude should be when carrying passengers.

The verbiage of the CFR is important (imagine that). The part you quote is really saying the following: if you want to fly an airplane which is equipped with a pressurization system, then, when the airplane is at its maximum operating altitude, the system must be able to keep the cabin pressure at or below 8,000 feet.
Nowhere in the reg you quoted is there anything about maintaining 8,000 feet, and in fact upon landing the dump valves will open and the cabin will depressurize to whatever the ambient pressure is anyway.
I imagine that airline crews would pressurize for passenger comfort until descent, and then gradually bleed away cabin pressure until touchdown. The aircraft will maintain 8000 feet until touchdown and then depressurize. Thanks @Kevin!

In the PC-12, there is a cabin pressurization controller knob (the lower right-hand knob in the image) that allows you to set the actual altitude at which you are taking off at/climbing or descending to/landing at.

Let's say you file for FL220. Before departure, you set the  cabin altitude on the dial to 22,000 ft. As the airplane climbs, the engine bleed air pressurizes the cabin until the maximum pressurization differential is reached (shown on the PSI DIFF gauge). The cabin climb rate is shown on the dial in the lower left corner.
When you descend to land, you set the knob to the destination field elevation + 500ft (the lower internal pressure is so you don't blow open the door!)
If the cabin pressure is 5,000 ft, and you're landing at a field that's 8,000 ft, the cabin will climb even as the aircraft descends.
If there is a failure of the pressurization computer, the pilot can adjust the cabin pressure manually using the knob in the upper righthand corner.
It's fairly simple to operate.

I've been waiting for an answer, but none forthcoming - so - this is GUESSWORK :)
The cabin altitude cannot be anything except the ambient altitude when the doors are opened before takeoff and after landing.
For landing, the cabin altitude would therefore be set to the airport altitude.  It cannot be anything else since there are valves which automatically prevent negative internal pressure.
For takeoff, my guess is that the cabin altitude would be set to normal according to SOPs for the aircraft and type of flight.  If the departure field was at a higher altitude, then this would result in an increase in cabin pressure.  Otherwise, the normal reduction in pressure would take place.


text:
What does an owner/operator need to do to legally rent out their aircraft?
I am interested in knowing all regulations apply to about what an owner operator needs to do to legally rent their aircraft out for private/business usage or flight instruction.

What inspections?
Insurance requirements?
Anything else?

I assume renting your aircraft out is under part 91.

Related: What are the legal requirements for me to purchase an airplane that I will provide instruction in?

Before considering rental, it would be good to check with the airport to find out about their policy. Some airports require you to have an office at the airport. e.g. Commercial activities.
No person shall establish or engage in any business or commercial activity on the airports unless that person has been assigned a fixed place of business on the airports and has obtained from the County either a written lease or a written permit to conduct such activity.

@OP, why do you think there are insurance requirements for legal operation?

As Farhan noted, the rules are essentially the same as if you buy a plane to provide instruction in it.
Inspections and Maintenance
In addition to your required annual inspection you will also be required to complete a 100-hour inspection every 100-hours of operation.
Insurance
You will need a commercial insurance policy which allows you to rent the aircraft.
This policy covers the aircraft, so you will also need (or at least want) general liability insurance and all the other trimmings that come with operating a business.
Everything Else
This is a broad category. There's no way we can cover it all. You will however need to consider:

Maintaining an adequate stock of "service spares" to maintain the aircraft.
Tires, light bulbs, oil, oil filters, 5606 hydraulic fluid, etc. - As an individual owner you may be OK with being down for a week waiting for parts, but as a rental operation that will lose you customers.
Having a relationship with a mechanic to do the maintenance
Owner-Performed Preventive Maintenance goes out the window when the aircraft becomes a rental - you may still be able to assist your mechanic, but you can no longer do & sign-off on things like oil changes.
Vetting your renters
Rental aircraft at an FBO/Flight School live a hard life. If you like your plane and want it to stay in good condition you'll need to vet your renters a bit more thoroughly than "You have a pilot's license and you went around the pattern without hitting anything" - Find the kind of pilots who will police their trash from the cabin, lean the mixture appropriately, etc.

Would you elaborate on the change in requirements of "Mandatory Service Bulletins"? Maybe a reference. I assume we're talking FAA here and I do not see that requirement in Part 91. I do see it in Part 135 but flight instruction and/or business use typically doesn't require 135 adherence.

@SteveH I thought it was required for Part 91 "for hire" ops (including renting the plane out) as well but I could very well be wrong - I'll double check the regs & update.

@SteveH A cursory review of 91.405 and a few ACs indicates you're correct: MSBs don't become mandatory for Part 91 Rental or "For Hire" ops. Updated the answer - thanks for catching it!

Thanks, as an inspector working for a repair station, I'm glad I wasn't missing something there.

Owner-Performed Preventive can still be performed by the owner. Part 43 does not restrict you in any way from performing maintenance on aircraft that you rent out. From a liability perspective, you may want to consider whether the risk is worth it since anyone who has ever touched an airplane is often sued when there is an accident.

100-hour inspections are not required per §91.409 Inspections unless you provide instruction or instructors. If you rent out the plane and the renters arrange for the instructor, you do not need 100 hour inspections.

This answer seems completely wrong. The 100-hr inspection is not a requirement, as amply detailed by others, and I have never heard of, nor can I find any FARs relating to, insurance requirements.

The top rated answer is incorrect.  If the aircraft is to be rented out to licensed pilots (i.e. not for instruction, but rather straight rental, just like renting a car) then the 100 hour inspections are not required.

Can you provide a citation for this?

The top-rated answer can change over time, it would be good if you can link directly to the answer (use the "share" link at the bottom). And as @fooot said, a link to the regulation for this would be great.

§91.409   Inspections. Specifically states that you only need to have 100 hour inspections if you “operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, ”. If you just rent out the aircraft and do not provide pilot services or instruction you do not have to get 100-hour inspections.

It would seem like an awfully good idea to ensure the plane was inspected anyway to catch whatever the renters you thought were golden actually messed up. Cheap insurance, and maybe it could lead to actually cheaper insurance?

The top answer is not correct. If you’re purely renting the aircraft out, it DOES NOT need a 100-hr inspection.
The 100-hour inspections are defined under FAR 91.409:
The 100-hour inspection is required for aircraft:

That carry any person (other than a crew member) for hire; or
That are provided by any person giving flight instruction for hire.

The phrase "for hire" refers to the PERSON, not the aircraft. The common FBO rental aircraft scenario always comes up. Ya, you're paying to rent the aircraft, but you're not hiring someone to fly it. Rental aircraft don't require a 100-hr.
Any operation that supplies both the flight instructor and the aircraft, requires a 100-hr. This is in contrast to a student pilot who provides their own aircraft and hires an instructor -- a 100-hr is not needed. This also falls in line with general commercial activities. Flight schools will routinely get to the 98ish hour mark, cease flight instruction in that plane and then allow it to just be rented until they can get it scheduled for their 100-hr. Happens all the time.

Hello Louie, would you happen to have some references to add to your post, so that we can turn this comment into a proper answer?

@Federico, updated the answer with context.